I was reading this article from Computerworld where many analysts opined about what will happen to Apple after the release of Chrome OS. And I agree  100 % with this opinion:

If…[adds] a tablet-like device to its lineup and dropping MacBook prices ($700), Gottheil concluded, "Then I don’t think Chrome OS is much of a threat."

In another hand, I think that Microsoft will feel more pressure and will be forced to create a Version of Windows more suited for the use in Small Factor PCs or what we know as Netbooks.